The invention relates to image detectors and more specifically, to a polarization analyzer bonded to a charge-coupled device (CCD), serial shift register and associated image transfer circuitry, analog signal processors, and analog-to-digital (A/D) converters, all integrated on a single silicon chip.
All solar activity arises from the action of the sun's magnetic fields. Precise measurements of these fields are essential for a complete description of the storage, release, and transport of energy in solar flares and in all the other energetic phenomena termed "solar activity". To understand solar activity, we must understand the electrodynamic and hydrodynamic processes that build up and store magnetic energy and release it. Explosive release, as in flares, probably occurs by the development of an instability in a sheared magnetic field. The best way to learn which processes are important to solar activity is to map the magnetic fields precisely for long intervals.
While a clearer understanding of the magnetic origins of solar flares would be an exciting development in solar physics, it will also bring important practical benefits. For example, manned exploration of space beyond the earth's magnetosphere will require substantial improvements in the reliability of solar activity forecasts. Without good forecasts, the freedom of astronauts to explore the moon and to carry out extravehicular activity (EVA) will be limited. Lethal or disabling protons from a flare can reach the moon in only 20 minutes after the first optical or X-ray emissions. Thus, whenever there is an active region on the solar disk, the astronauts must be able to reach shelter quickly. One-to-three-hour forecasts could greatly expand the scope of astronaut EVA. Such forecasts would probably have to be based on solar magnetic field maps and the detection of magnetic energy build-up.
Currently, solar vector magnetographs (e.g., Rust, et al. U.S. Pat. No. 5,125,743) are one type of instrument used to produce maps of the sun's magnetic fields. Such maps, called magnetograms, are assembled from precise measurements of the polarization of sunlight, in spectral lines sensitive to the Zeeman effect. Unfortunately, solar vector magnetographs are notoriously complex instruments incorporating rotating or scanning mechanisms often having multiple beams with separate detectors whose outputs must be corrected and correlated before image analysis can begin. An added complication is solar image distortion caused by turbulence in Earth's atmosphere. Even when single image pairs of an event are taken only a fraction of a second apart, that time difference can seriously affect polarization measurements.
In another area of scientific interest and study, the Earth Observing System (EOS) is an important component of the national research thrust to learn more about our environment. The EOS and other earth-observing missions will need to map atomic and molecular emissions with unprecedented spectral and spatial resolution. To understand the chemistry and transport of molecular species in the middle atmosphere, the concentrations of O.sub.3, N.sub.2 O, CH.sub.4, CFCl.sub.3, CF.sub.2 Cl.sub.2, HCl, HNO.sub.3, H.sub.2 O.sub.2, HNO.sub.4, ClONO.sub.2, etc., must be mapped with at least 10% accuracy. Such mapping will require precision spectroscopy of the molecular lines.
Other requirements for an imaging spectral radiometer are emerging in the study of the energetics of the upper mesosphere and lower thermosphere, where non-LTE (Local Thermodynamic Equilibrium) excitation is important. Emission line profiles for OH, NO and CO.sub.2 need to be studied with high time and spatial resolution to determine the composition and temperature structure of the region. Narrow-band spectral images are required to measure the spatial variation of the outgoing energy.
In another application, polarimetric measurement of light reflected from clouds and ground features provides important information. For example,
a. The thermodynamic phase of water droplets and ice crystals in cirrus clouds might be determined from photopolarimetry. PA1 b. Polarimetric imagery provides a potentially rich source of data on flowering and water :stress in corn and soybean crop canopies. PA1 Dual interleaved 512.times.1024-pixel CCDs provide simultaneous data collection in two polarization planes at 10 Mpixels/sec from each output port. PA1 A deep-well buried-channel CCD design with custom doping profiles provides 10.sup.6 electron storage capacity per pixel. This will allow a precision of 1 part in 10.sup.3 per frame and 1 part in 10.sup.4 in 100 frames, when the data are accumulated and averaged externally to the IDID. The imaging rate can be as high as 72 frames/sec. PA1 A customized charge-collection cell design will allow the light-collecting area to be .about.85% of the geometrical area. PA1 Bonding the rutile (TiO.sub.2) polarizing beamsplitter to the CCD eliminates rotating polarizers and retarders, and separate light paths for orthogonally polarized beams. The rutile beamsplitter allows use of a common light path before the beams are split for analysis, and it thereby eliminates the misalignment and image scale differences encountered when multiple detectors are used. The bonding of the beamsplitter and imager achieves a rugged device which, after initial calibration in the laboratory, will provide polarization measurements that are limited in precision only by photon statistics. PA1 Smearing and loss of precision is avoided by not clocking the CCD during image capture. PA1 Adjacent columns of pixels sense the two planes of polarization at each point in the image to minimize detector gain differences between the two collected images. PA1 On-chip signal processing simplifies post-processing hardware and software. PA1 On-chip A/D converters provide 5 parallel 10-bit digital outputs at high speed, i.e., 10 Mpixels/sec. Low impedance analog outputs of all quantities are also provided.
In remote sensing, the precision witch which polarization can be determined is frequently limited by instrument or subject motion, or distortion by atmospheric turbulence on the time scale of the measurement. To fully characterize the state of polarization of incoherent light, at least five measurements must be made and these are usually obtained sequentially. Even at TV rates (30 images/sec), small scene changes between successive frames can seriously degrade polarization measurements, which are derived by subtracting successive images. For real-time applications, an added problem is the high cost and complexity of data processing equipment that can keep up with a stream of TV images.
In one very fast polarization and spectral image analyzer (See Lites, B. W., et al, 1991, Preliminary Results from the HAO/NSO Advanced Stokes Polarimeter Prototype Observing Run, in Solar Polarimetry (L. November, ed.), Sunspot, NM, National Solar Observatory, pp. 3-15.), the image plane of a telescope is scanned with a narrow slit, and the transmitted light is analyzed with several CCDs in a spectrograph. Each CCD operates at 60 frames/sec, and each pixel is digitized to 12-bit precision. To acquire data on orthogonal polarization states simultaneously, this instrument uses a polarizing beamsplitter to illuminate two CCD detectors for each wavelength of interest. The technique provides very high precision, but the complexity of the device and the use of paired detectors, which are difficult to align and intercalibrate, renders it unsuitable for use in space or other harsh environments.
Another device has been able to remove the problem of pixel-to-pixel intercalibration in polarization measurements by using the same pixels for both polarized images. (See Povel, H. P., et al, 1991: Two-Dimensional Polarimeter with CCD Image Sensors and Piezo-Elastic Modulators, in Solar Polarimetry (L. November, ed.), Sunspot, NM, National Solar Observatory, pp. 102-112.) The device masks every other row of a custom-designed CCD so that two images, in alternating phases of polarization, can be stored in the CCD by cycling the two sets of accumulating charge packets in and out of the masked rows in synchronism with the polarization modulator, which operates at 50 kHz. The fast modulation removes the effects of almost any conceivable image motion or distortion. A drawback of the scheme is that it makes very inefficient use of the light.
In short, current spectroscopic imaging polarimeters are too complex. Furthermore, their speed and field of view are frequently too limited to record subtle developments, especially where the spectral lines to be detected differ only very slightly from the background illumination.
What is needed then is an imager which is greatly simplified in design and operation and which is capable of obtaining and precisely registering simultaneous images of a rapidly changing, low contrast scene.